1. Field of the Invention
The present invention relates to a method of coupling a guided wave from an optical waveguide through a grating coupler or coupling external light into an optical waveguide through a grating coupler. The present invention also relates to an optical pickup for reading signals recorded on optical recording mediums such as optical discs or on magnetooptic recording mediums, and more particularly to an optical pickup employing an optical waveguide.
2. Description of the Prior Art
Some conventional optical waveguides include grating couplers mounted on the surface of the waveguide. The grating couplers serve to introduce an external light beam into the optical waveguide and extract a guided wave from the optical waveguide, i.e., to couple an external light beam with the optical waveguide or a guided wave which is propagating in the optical waveguide to an external device. A grating coupler is more advantageous than other light input and output coupling devices such as prism couplers since grating couplers contribute to smaller and lighter optical waveguides.
However, the grating couplers are problematic in that when they are used to couple external light into optical waveguides, the efficiency with which the external light is introduced into the optical waveguides (hereinafter referred to as the "light input efficiency") greatly varies depending on changes in the wavelength of the external light, and when they are used to couple guided waves from optical waveguides to other devices, the angle at which the output light leaves the optical waveguides (hereinafter referred to as the "light exit angle") varies greatly depending on changes in the wavelength of the light.
In most waveguides, a laser beam emitted from a semiconductor laser is what is guided. Since a semiconductor laser beam is susceptible to wavelength fluctuations as is well known in the art, it is necessary to employ a temperature regulating device such as a Peltier-effect device, which is relatively expensive, to regulate the temperature of the semiconductor laser in order to suppress variations in the wavelength of the laser beam emitted by the semiconductor laser. However, if the laser beam of a semiconductor laser is directly modulated, then it is highly difficult to reduce wavelength fluctuations sufficiently even if such a temperature regulating device is used.
FIG. 15 of the accompanying drawings illustrates the manner in which the light input efficiency .DELTA. varies depending on the change .lambda..eta. in the light wavelength. It is assumed that the wavelength (i.e., the central wavelength which is used as the reference wavelength when a grating coupler is designed) of the external light to be applied to an optical wavelength is .lambda.=788 nm, the effective refractive index of the optical waveguide is N=2.187, the refractive index of the substrate of the optical waveguide is Ns=2.182, the period of the grating coupler is .LAMBDA.=3.00 .mu.m, the angle at which the external light is applied to the grating coupler is .PHI.=61.87.degree., and the length along which the external light is coupled through the grating coupler to the optical waveguide (hereinafter referred to as the "coupling length") is b. The variation of the light input efficiency is expressed by .eta./.eta..sub.o where .eta..sub.o is the maximum light input efficiency. As can be understood from FIG. 15, even if the coupling length b is small enough for intensive light coupling, the light input efficiency .eta. will be reduced to about 1/2 of the maximum light input efficiency .eta..sub.0 when the light wavelength varies by about 2 nm, for example.
In cases where a guided wave is extracted from an optical waveguide through a grating coupler on the surface thereof, if the period of the grating coupler is .LAMBDA.=3.78 .mu.m and the light exit angle is 65.degree., then the light exit angle varies by about 0.63.times.10.sup.-3 per light wavelength change .DELTA..lambda.=1 nm.
In addition, the grating couplers are also disadvantageous in that the light input efficiency thereof, when used to couple external light into optical waveguides, and the light exit angle thereof, when used to couple guided waves from optical waveguides to external devices, are highly sensitive to changes in the ambient temperature. Such high sensitivity to temperature variations will be described below with reference to a grating coupler which is used to couple external light to an optical waveguide.
When external light is applied through an optical waveguide substrate to a grating coupler, the phase matching condition, i.e., a condition which must be met for phase matching to be achieved between the external light and a guided wave in the optical waveguide, is given by: EQU n k sin.PHI.=Nk-mK (1)
where .PHI. is the angle of incidence of the external light upon the grating coupler, N is the effective refractive index of the optical waveguide, m is the coupling order number of the grating coupler, and k and K are parameters defined as follows: EQU k=.vertline. .vertline.=2.pi./.lambda. EQU K=.vertline. .vertline.=2.pi./.LAMBDA.
where and are the wave vectors of the light and the grating coupler, respectively, .lambda. is the wavelength of the light, and .LAMBDA. is the period of the grating coupler. When the external light is applied to the grating coupler through the substrate as described above, it is normally introduced through an obliquely cut end surface of the substrate. In most cases, the angle of incidence of the external light upon the end surface of the substrate is set at 0.degree., i.e., the external light is applied perpendicularly to the end surface of the substrate, in order to facilitate the design of an antireflection coating on the end surface.
Generally, the refractive index n of a substrate material has a high temperature coefficient. For example, the temperature coefficient of the refractive index .DELTA.n/.DELTA.t of LiNbO.sub.3 is 5.3.times.10.sup.-5z deg.sup.-1. Since the effective refractive index N of an optical waveguide varies depending on the change in the refractive index of the substrate thereof, phase matching is gradually degraded and the light input efficiency decreases as the temperature changes when the incident angle .PHI. remains constant, as is apparent from equation (1) above.
When external light is introduced through the end surface of the substrate, if it were not applied perpendicularly to the end surface, the light exit angle from the end surface would vary depending on the change in the temperature, i.e., the refractive index, of the substrate, and the angle of incidence .PHI. of the external light upon the grating coupler would vary. The phase matching condition cannot be satisfied at all times at each temperature if only the incident angle .PHI. varies.
When external light is applied to the grating coupler through a surrounding medium (normally air) opposite to the substrate of the optical waveguide, the phase matching condition is given by equation (1) except that the refractive index n of the substrate of the optical waveguide is replaced with the refractive index of the surrounding medium. With this arrangement, too, the effective refractive index N of the optical waveguide varies depending on the temperature change of the substrate, and since the angle .PHI. of incidence of the external light upon the grating coupler remains constant, the same problems mentioned above occur.
As described above, the light input efficiency varies depending on the temperature change of the substrate when a grating coupler is used to couple the external light to the optical waveguide. It is clear from the reciprocity theorem about input light and output light that the light exit angle varies depending on the temperature change of the substrate when the guided wave is extracted from the grating coupler.
Optical recording mediums such as optical discs have recently been widely used for recording image signals and audio signals. Signals are recorded on optical recording mediums in the form of pits or as different reflectivities, and can be read by an optical pickup. Hereinafter, it is assumed that signals are recorded as pits in an optical recording medium. The optical pickup applies a light beam such as a laser beam, for example, to the recording surface of the optical recording medium, and detects the intensity level of the light reflected from the optical recording medium to determine whether there is a pit or not at the spot where the light beam is applied.
In addition to the reading of the recorded information, the optical pickup also detects tracking errors and determines whether a light beam used to detect pits has been displaced laterally from the center of a series of pits (track). The optical pickup also detects focusing errors and determines whether a light beam is focused properly, i.e., whether it is overfocused or underfocused, with respect to the recording surface of the optical recording medium. Tracking and focusing error signals are used by tracking and focusing feedback control loops to align the light beam with the track and also to focus the light beam properly on the recording surface of the optical recording medium, until finally the tracking and focusing error signals are eliminated. Conventional tracking error measuring methods include a push-pull method, a heterodyne method, and a time-difference detecting method, and conventional focusing error detecting systems include an astigmatic method, a critical-angle detecting method, and a Foucault knife-edge method.
Magnetooptic recording mediums such as magnetooptic discs have also been widely used as mediums for recording image signals and audio signals. The direction of magnetization in the magnetooptic recording medium carries the information about the signals recorded on the medium, and the magnetization direction can be read by an optical pickup. The optical pickup applies a linearly polarized light beam such as a laser beam, for example, to the recording surface of the magnetooptic recording medium, and detects the direction of magnetization in the magnetooptic recording medium, based on the principle that the plane of polarization of light reflected from the recording medium will rotate depending on the direction of magnetization. (The phenomenon is known as the magnetooptic Kerr effect.)
More specifically, an optical pickup for use with a magnetooptic recording medium has a light detector for detecting light reflected from the recording medium through an analyzer. Since the intensity of the reflected light varies as its plane of polarization rotates, the optical pickup can detect the direction of magnetization, i.e., the information recorded on the recording surface of the magnetooptic recording medium. An optical pickup of this type also has tracking and focusing error detecting systems in addition to its ability to read recorded information.
Heretofore, conventional optical pickups have included a beam splitter for separating the light beam reflected by an optical or magnetooptic recording medium from the light beam being applied to the recording medium, a lens for focusing the reflected light beam onto a light detector comprising a photodiode or the like, and a prism for directing the reflected light beam toward the tracking and focusing error detecting systems. An optical pickup for use with a magnetooptic recording medium also includes microoptic devices such as an analyzer, as referred to above.
Microoptic devices require precise machining when they are produced, and it is tedious and time-consuming to adjust their positions with respect to each other when they are incorporated in an optical pickup. Therefore, optical pickups which include such microoptic devices are expensive to manufacture. Since these optical pickups are relatively large and heavy, information read-out units including them are limited as to how small and light they can be, and the speed with which desired information recorded on a recording medium can be accessed is also limited.
In view of the aforesaid drawbacks, there have been proposed optical pickups which are small and light and can be manufactured inexpensively because they employ optical waveguides.
One of the proposed optical pickups, which is used with an optical recording medium, includes a single optical waveguide device having convergent grating couplers. The optical waveguide device is capable of functioning as a beam splitter, a lens, and a prism, as described above.
More specifically, the optical pickup comprises a light source for applying a light beam to the recording surface of an optical recording medium such as an optical disc, an objective lens for focusing the light beam onto the recording surface of the optical recording medium, and an optical waveguide directed such that it receives, on one surface thereof, the light beam reflected from the recording surface of the optical recording medium.
The optical pickup also has first and second convergent grating couplers on the surface of the optical waveguide, to which surface the reflected light beam is applied, for introducing the reflected light beam into the optical waveguide. The first and second convergent grating couplers are disposed one on each side of an axis which passes substantially through the center of the reflected beam applied to the optical waveguide and also which extends across the surface of the optical waveguide in a direction which is perpendicular to a normal of a track of recorded signals. The first and second convergent grating couplers are arranged such that they converge the reflected light beams propagating in the optical waveguide toward positions spaced from each other across the axis.
The optical pickup further has first and second sets of light detectors which are disposed on the surface or an end face of the optical waveguide and detect the reflected light beams respectively converged by the first and second convergent grating couplers. Thereby the optical pickup reads recorded information and detects any tracking and focusing errors from the reflected light beams. For further details, reference should be made to Japanese Unexamined Patent Publication No. 63(1988)-61430, for example.
The other optical pickup proposal, which is used with a magnetooptic recording medium, includes a single optical waveguide device which has convergent grating couplers and is capable of functioning as a beam splitter, a lens, and a prism, as described above. This optical pickup also includes a semireflecting mirror for differential detection of recorded information.
More specifically, the optical pickup comprises a light source for applying a linearly polarized light beam to a recording surface of a magnetooptic recording medium such as a magnetooptic disc, an objective lens for focusing the light beam onto the recording surface of the magnetooptic recording medium, and an optical waveguide directed such that it receives, on one surface thereof, a light beam reflected from the recording surface of the magnetooptic recording medium.
The optical pickup also has first, second, and third convergent grating couplers on the surface of the optical waveguide, to which surface the reflected light beam is applied, for introducing the reflected light beam into the optical waveguide. The first and second convergent grating couplers are disposed one on each side of an axis which passes substantially through the center of the reflected beam applied to the optical waveguide and also which extends across the surface of the optical waveguide in a direction which is perpendicular to a normal of a track of recorded signals. The first and second convergent grating couplers excite the light beams and cause them to travel in a TE or TM guided mode, and are arranged such that they converge the reflected light beams, which are propagating in the optical waveguide in the same guided mode, toward positions spaced from each other across the axis.
The third convergent grating coupler excites the light beams and causes them to travel in a guided mode different from the guided mode excited by the first and second convergent grating couplers (e.g., a TM guided mode when a TE guided mode is excited by the first and second convergent grating couplers), introduces the reflected light beam into the optical waveguide, and converges the propagating light beam in the optical waveguide.
The optical pickup further has first, second, and third sets of light detectors, which are disposed on the surface or an end face of the optical waveguide and detect the reflected light beams respectively converged by the first, second, and third convergent grating couplers. The optical pickup also includes detecting circuits for detecting tracking and focusing errors based on output signals from the first and second light detectors, and a differential detecting circuit for detecting recorded information based on the difference between the output signals from the first and/or second light detector and an output signal from the third light detector. For further details, reference should be made to Japanese Unexamined Patent Publication No. 63(1988)-188844, for example.
In the above optical pickup, the functions which have heretofore been performed by the beam splitter, the lens, the prism, the semireflecting mirror, and the analyzer are performed by the convergent grating couplers. Therefore, the optical pickup is composed of a highly reduced number of parts, is small and light, can be manufactured much less expensively than the conventional optical pickups, and can access desired recorded information in a shortened access time. Particularly, the cost of the optical pickup is greatly reduced because its major component can easily be mass-produced by the planar process.
With an optical pickup employing an optical waveguide, nevertheless, inasmuch as a light beam reflected from a recording medium is introduced into the optical waveguide through a grating coupler on the surface of the optical waveguide, the light input efficiency tends to vary due to the wavelength selectivity of the grating coupler, as described above.
If a semiconductor laser is employed as a light beam source, a high-frequency current is added to the drive current supplied to the semiconductor laser so that the semiconductor laser will operate in a multiplicity of modes and noise caused by the reentering light will be reduced. When the semiconductor laser is operated in this manner, the light output efficiency may drop sharply because the reflected beam entering the grating coupler has wide range of wavelengths.
When the light input efficiency varies, the intensity of light detected by the light detectors also varies irrespective of the recorded information and the tracking and focusing conditions of the light beam applied to the recording medium. As a result, errors may occur when the recorded information is read, and tracking and focusing errors may be detected improperly.